Why would $env:username and [environment]::username return different users?

Question

What is the difference between PowerShell's $env:username and [environment]::username and why would they potentially return different users? (I understand there are other ways to get the current user as well)

Some background:

I have an Azure Pipeline that runs a PowerShell script on a release target. The pipeline agent is configured to run under a specific service account. Part of that PowerShell script uses $env:username to assign permissions. However, permissions are assigned to the local admin account instead. If I change the script to use [environment]::username the correct service account user is given permissions.

Answer 1

$env:USERNAME, while predefined to reflect the current user's username, is a read-write environment variable, just like any other.

Even though it's obviously inadvisable to do so, a statement such as $env:USERNAME = 'foo' changes the value of environment variable USERNAME for the current process as well as its child processes.

This means that if environment variable USERNAME was modified earlier in the same session or in PowerShell's parent process, possibly via an explicitly specified startup environment, it no longer reflects the true username.

While I don't know why the USERNAME environment variable would differ from the real account in the case of Azure Pipelines, an example of - inadvertently - setting the wrong value is PowerShell's Start-Process cmdlet when given the -UseNewEnvironment switch: due to a bug as of PowerShell Core 7.0.0-preview.5 / Windows PowerShell v5.1, $env:USERNAME unexpectedly always reflects SYSTEM, irrespective of the actual user account - see this GitHub issue.

By contrast, [Environment]::UserName uses a different method for obtaining the current user's username, which does not rely on the value of $env:USERNAME and always reflects the true username^[1].

In short: Only [Environment]::UserName reliably reflects the current user account's username.

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^{[1] From the linked docs: "On Windows the UserName property wraps a call to the Windows GetUserName function. The domain account credentials for a user are formatted as the user's domain name, the \ character, and user name. Use the UserDomainName property to obtain the user's domain name and the UserName property to obtain the user name.
On Unix platforms the UserName property wraps a call to the getpwuid_r function.}